Now when you undock Windows notices and puts all applications onto your single primary screen (the laptop screen).
When you re-dock Windows again notices and says "Ahhhh - Bob wants Application X on screen two" and all is well with the world.
One day Bob can't sit at his usual desk and instead docks onto a different but identical docking station which only has one screen. He the loads Application X
Windows notices that we are at a docking station that it recognises but doesn't appear to understand that there is only a single screen and as such loads Application X onto a non-existent screen two.
To move it back:
2) Press ALT + Space together
If Application X is full screen on screen two then select Restore then redo this step
3) Use the cursor keys to move down to move and press Enter
4) Press a direction on the cursor key to make Application X stick to the mouse
5) Move the mouse to drag Application X onto the working screen